Civil Engineering Reference
In-Depth Information
c
Anisotropic phase
Solid
Nematic phase
liquid crystal
liquid crystal
liquid
T
Figure 14.2
Schematic diagram of molecules orientation in solid, liquid crystal and liquid states.
Concentration (c) and temperature (T) alternate upwards and downwards, respectively [76].
Cellulose and its derivatives have the ability to behave both as thermotropic and
lyotropic liquid crystals. As mentioned above, several specii c phases of liquid crys-
tals occurs, depending on the structure or combination of molecules. In the nematic
phase, the molecules have only orientational ordering (making the liquid crystal phase
less ordered), while in the smectic phase, the molecules have both orientational and
positional ordering [75]. In addition, the optically active molecules can form a chiral
nematic phase (or cholesteric phase). In this case, the molecules are helix-oriented gen-
erating some spectacular optical properties.
Demus, Goodby and Gray [16] present dif erent systems based on cellulose deriva-
tives, in which the ordered cholesteric liquid crystalline phase appears only in specii c
solvents at high concentrations, remaining isotropic in other solvents (Table 14.1). In
addition, one can notice that the l exibility of side-chain substituents can generate ori-
entational equilibrium of the main chains and also that the numerous large substitu-
ents from the cellulose backbone increase the hydrodynamic volume, inl uencing chain
conformation.
On the other hand, literature data show [16] that dif erent cellulose derivatives
which form liquid crystalline solutions in organic solvents may also form cholesteric
thermotropic phases in the absence of a solvent—with spontaneous molecular orien-
tation and cholesteric rel ection, such as: 2-acetoxypropyl cellulose, 2-hydroxypropyl
cellulose, the tril uoroacetate ester of hydroxypropyl cellulose, the propanoate ester of
hydroxypropyl cellulose, the benzoate ester of hydroxypropyl cellulose, 2-ethoxypropyl
cellulose, acetoacetoxypropyl cellulose, tril uoroacetoxypropyl cellulose, the phenylac-
etate and 3-phenylpropionate of hydroxypropyl cellulose, phenylacetoxy, 4-methoxy-
phenylacetoxy, p-tolylacetoxy cellulose, trimethylsilyl cellulose, trialkyl cellulose,
cellulose trialkanoate, the trialkyl ester of (tri-o-carboxymethyl) cellulose, 6-o-
-(l-
methylnaphthalene)-2,3-o-pentyl cellulose, etc. Moreover, the suspensions of cellulose
crystallites spontaneously form the chiral nematic phase. h e formation of mesophase
suspension of cellulose crystallites varies from one type of cellulose to another, being
inl uenced, in the formation of the chiral nematic phase, by the mineral acid selected
α
